Ongoing research in the area of sustainable solutions to alleviate petrochemical dependence includes solar fuels, the effort to synthesize liquid fuel materials based on harnessed solar energy. This work is important, because it enables us to provide commodity chemicals and energy carriers without reliance on petroleum products. Many of these strategies involve electrocatalytic (or photocatalytic) cleavage of water to form hydrogen and oxygen. The reducing equivalent, H2, is thus an energy carrier because it can be re-oxidized, either by combustion to give heat or catalytically in a fuel cell to give electricity. There is a disabling problem with large-scale utilization of hydrogen as a fuel, since it is a gas under ambient conditions, thus limiting its volume-energy density (0.013 MJ L−1). As a result, physical-based hydrogen storage technologies (compression, cryogenic liquefaction, adsorption) involve low capacity, high costs, or safety issues. Therefore, the discovery of highly weight-efficient strategies for on-demand hydrogen release from hydrogen-rich liquids has potential value toward enabling hydrogen (with an appropriate H2 oxidation fuel cell) as a renewable fuel for light vehicles. Formic acid (HCO2H, FA, 7.5 MJ L−1) is a hydrogen carrier, owing to its ability to release hydrogen under mild conditions with only CO2 as a by-product, which can then be recycled, in principle, to give a carbon-neutral fuel cycle.
To date, many efficient homogeneous and heterogeneous catalysts for FA dehydrogenation have been developed. Heterogeneous catalysts have advantages of separability and reusability, while homogeneous catalysts are generally more efficient, the latter exhibiting the best turnover number and turnover frequency. Moreover, homogeneous catalysts generally are more selective, producing less carbon monoxide, a common byproduct of FA dehydrogenation. This is essential, because CO is a fuel cell catalyst poison. Still, no known system is stable and reactive through multiple uses, air and water tolerant, selective against CO formation, and functions in neat formic acid liquid. Each of these is critical to achieving a usable hydrogen generation system based on formic acid. Herein we report a novel iridium-based catalytic system that meets all of these criteria.
Accordingly, there is a need for new catalyst systems for reducing formic acid and related compounds.